Leo played 11 seasons in the minors, winning in double figures several times. Most of his minor league career was spent in the Tigers organization. He was on the 1967 Toledo Mud Hens team that won Toledo's first Governor's Cup Championship. He is part of Toledo team photos included in the book Baseball in Toledo. He was originally signed by the Tigers out of a local high school before the 1960 season and made his first two major league appearances with them at the end of the 1965 season, pitching three scoreless innings.

Marentette, who pitched on Canada's first major league team, also pitched for the Vancouver Mounties and Winnipeg Whips. He was one of three pitchers with major league experience acquired by the Montreal Expos on the eve of their inaugural season, in 1969, along with Steve Shea and Howie Reed, as the last part of the convoluted Rusty Staub trade. Shea began the season in Montreal, while Reed and Marentette were sent to AAA Vancouver, which was co-operated with the Seattle Pilots, one of the Expos' brothers in expansion. Vancouver was one of the teams that Jim Bouton pitched for that year and as a result, Leo gets a passing mention in Ball Four. He was called up by the Expos on June 1st, replacing Shea, and stayed on the roster for a couple of weeks, until the Expos signed veteran Dick Radatz on June 16th. The team was in the middle of franchise-record 19-game losing streak when Leo was called up, and he pitched three times, giving up 4 runs on 9 hits in 5 1/3 innings. All three of his appearances were on the road, so he never had a chance to be introduced to hometown fans, where his French-sounding first and last names would doubtless have made him quite popular. In Vancouver, he was 3-2, 2.03, in 20 games, with 6 saves; he also got to pitch again for the Mud Hens that year, on loan to the Tigers' organization, but was 1-3, 7.50 in 17 games.

In 1970, he pitched briefly for the Buffalo Bisons, which became the Winnipeg Whips a few weeks into the season. He was 0-1 with an ERA of 9.00 in 9 games, ending his professional career.

After baseball he worked for a while for a trucking firm in Toledo, OH and later lived nearby in Michigan. He died there of a heart attack in 2014, at the age of 73.